There are a number of key reasons NOT to use Windows Server 2008 R2 for AlwaysOn Availability Groups (AGs), including:

It’s an older operating system—released nearly 4.5 years ago. A lot has changed in 4.5 years. Windows Server 2012 and 2012 R2 may come with the metro interface—which is a bit stupid to put on a server—but they’re much faster, more stable, and more secure operation systems by a wide margin and also picks up vastly improved SMB (as an example of some of the improvements you can expect).

Windows Server Failover Clustering (WSFC) wasn’t mature enough to handle AGs. Availability Groups may not be as hard to set up and configure as AlwaysOn Failover Cluster Instances (FCIs) or traditional clusters, but they’re still based upon WSFC. As you know, WSFC on SQL Server 2008 R2 simply wasn’t mature enough to tackle some of the use cases encountered by AGs—hence some of the initial horror stories about spectacular AG failures on Windows Server 2008 R2, along with the need to track down and install gobs of hard-to-find hot-fixes/patches on Windows Server 2008 R2 before CORRECTLY and SAFELY deploying Availability Groups.

It is, in most cases, MORE EXPENSIVE to license. Failover Clustering on Windows Server 2008 R2 was only available as part of the Enterprise Edition (or higher) SKUs—much more expensive to license than Windows Server Standard Edition. With Windows Server 2012 and above, Standard Edition now picks up FULL support for Clustering (not just simple 2-node active/passive Clusters only—but full-blown Clustering support).

Windows Server 2012 R2 DataCenter Edition picks up a roughly 28 percent price increase, but Standard Edition stays roughly the same price. The only caveat, of course, is that Windows Server 2012 and above, need to be licensed via 2-Processor Packs—which is a pain for smaller servers—and that CALs are no longer included in most pricing options as part of the license for Windows Server 2012 and above. Or, in short, licensing Windows is always complex, convoluted, and cryptic enough that you typically need to spend a solid 20 minutes perusing documentation before you can actually feel that you get a semblance of a grip on what you need to buy/license to provision new servers.

WS 2012 R2 Standard Edition is Great for Availability Groups

The point I’m trying to get across, though, is that in most environments, Windows Server 2012/2012 R2 Standard Edition is enough to keep SQL Server AlwaysOn Availability Groups adequately and amply powered and happy. In fact, the only differentiator between the DataCenter Edition and the Standard Edition is that DataCenter Edition provides unlimited hosting of virtual machines, or VMs (i.e., license the physical processors and run as many VMs as your hardware can handle) whereas, Standard Edition allows only 2 VMs per License 2-Pack (with any further VMs requiring their own, actual, licenses), and is really geared more towards traditional or physical workloads—which is what you're going to want in most cases for your AGs, anyhow.

Other than that, BOTH versions of Windows Server support the same amount of hardware, throughput, and are full-featured versions of Windows Server (i.e., they’re not artificially restricted in any way).

Contributors

Michael K. Campbell is a contributing editor for SQL Server Pro and Dev Pro and is an ASPInsider. Michael is the president of OverAchiever Productions, a consultancy dedicated to technical evangelism...